In the field of automatic preparation and dispensing of espresso coffee, known apparatuses are described for example in EP-A 0 154 206 and in EP-A 1 306 041.
As is known, espresso coffee is prepared by forcing hot water, normally at a temperature of between 88° C. and 95° C., to pass through a layer of ground coffee, which will be indicated hereinafter as powdered product or simply as coffee powder.
In order to carry out the process of preparing the beverage, the chamber, containing the powdered product, must be hermetically sealed to allow the pressurisation of the water which has to pass through the layer of powder and, consequently, one of the characteristic technical problems which face the designer of a coffee machine is that of ensuring adequate and reliable closure of the chamber in which the process of preparation of the beverage takes place.
The spread of espresso coffee machines in the world has led to diversification of the types of this beverage which have been influenced by the taste and tradition of the various countries.
Therefore, the quantity of coffee powder used and the volume of the beverage dispensed into the cup may be very varied. For example, to prepare a “ristretto” espresso of the Mediterranean type of 15 ccm, an average of 6 grams of ground coffee are used, whereas to prepare a dose of coffee of 120-150 ccm as consumed in Northern Europe up to 18 grams of ground coffee may be required.
However, since the pressure at which the water is forced through the coffee powder and the extraction time for the beverage should remain as constantly as possible at the ideal nominal value respectively of 9 bar and 25 seconds, the optimisation of the process of preparation of the beverage in all situations comprised between the two extremes of doses indicated above, involves the adaptation of the fineness of grinding, and of the volume and the diameter of the infusion chamber.
The dimensions of the diameter of these chambers, generally cylindrical in shape vary, according to the type of coffee, between 35 mm and 50 mm.
This means that with a pressure of 9 bar there may be, on the members which effect the closure of the infusion chamber, forces of between 880 and 1800 N in the direction of their axis.
In the dispensing devices of modern automatic type coffee machines which are normally actuated by electric motors, the movements of opening and closing of the infusion chamber are devised in such a way as to prevent the reversibility of the movement under the action of the force resulting from the pressure exerted by the water during the preparation of the beverage.
Normally, the prior art seeks to obtain this result through stops of a mechanical type in order to avoid maintaining under stress the motors of the actuating means which would otherwise have to be over-sized in order to withstand prolonged and heavy service.
In order to obtain a good quality dispensed beverage it is also necessary that, before being subjected to the thrust of the pressurised water, the coffee powder is adequately compacted inside the infusion chamber in such a way as to form a firm layer which has a homogeneous resistance to the water, which should pass through it without forming preferred paths.
However, it is desirable that the force with which the dose of coffee powder is compressed is not too great, otherwise there is a risk of forming a layer so compacted that it forms a barrier to the water.
Normally, a force is selected which is such as to provide a pressure of around 0.15 MPa on the member arranged to effect the compression of the dose of powder.
Moreover, in order to obtain correct extraction of the aromatic substances typical of the beverage, the grains of ground coffee should absorb the water which is forced to pass through them and they should be able to increase in volume.
For this it is necessary that the member which effects the compaction, which is generally formed by the same piston which performs the function of closing the infusion chamber, once compaction is completed should be able to retract by an amount sufficient to permit the expansion of the firm layer of ground coffee.
According to the prior art, described for example in EP-A-1 800 574, the dispensing units are provided with infusion chambers in which the operating volume is determined when the piston is in the closure position with the connecting rod and crank in alignment. With such technology the chamber may contain at maximum a certain quantity of ground coffee which depends on the diameter of the chamber and on the dimensions of the means for moving the piston, i.e. of the connecting rod and crank. The compressive force exerted on the dose of coffee powder loaded into the chamber is provided by the resilient force of a spring interposed between the end of the connecting rod and its point of connection to the piston. Since the resilient force possessed by such a spring is less than the force generated by the pressurised water when it is admitted into the chamber, the piston rises up and allows the ground coffee to absorb the water and to expand.
Document EP-A-1 306 041 discloses a movement device for infusion dispenser assemblies, particularly for espresso coffee machines, which comprises a dispenser body that forms an infusion chamber and is supported by a supporting frame, a lower piston or dispensing piston being movable in said infusion chamber, an upper piston or infusion piston being insertable in said infusion chamber, and means for actuating said lower piston which are suitable to guide positively the translational motion of said lower piston during the upward or expulsion stroke and during the descent or return stroke.
Once dispensing of the beverage is completed, the flow of pressurised water is stopped and the residual water must be eliminated from the chamber and the exhausted grounds expelled. Generally, the residual water is eliminated by placing the chamber in communication with a discharge line while the resilient means itself which, once the thrust of the pressure has ceased, returns to compact the grounds and presses out the water. When sufficient time has elapsed to obtain adequate drying of the grounds, they are then expelled in order to prepare the chamber for any new dispensing. This type of dispensing unit, designed so as to effect the loading and closure of the chamber during the first half turn of the crank so as to carry out dispensing at bottom dead centre, are designed so as to perform the expulsion movements by advancing the crank beyond bottom dead centre in order to execute a complete revolution which brings the mechanism back into the initial state, ready to carry out a fresh cycle.
Although this is quite an efficient technique, it is however not devoid of drawbacks in the case where it is desired to increase the dose of ground coffee to be inserted into the infusion chamber. In that case it is necessary either to modify the diameter of the chamber or make it much longer, increasing the stroke of the piston, which solution entails the modification of the length of the connecting rod and of the crank.
The Applicant has observed that, when it is wished to produce a machine which, based on the above-mentioned technology, enables different types of coffee to be dispensed by using doses of powdered product of between 6 and 18 grams, once the cross-section of the infusion chamber is established, it becomes necessary to make its length such as to contain the product up to the maximum quantity by adjusting the stroke of the connecting rod and crank.
However, operating in this way it is possible to obtain the alignment of the connecting rod with the crank, in such a way as to maintain the axially aligned thrusts and avoid the occurrence of torque on the drive shaft which must be overcome by the motor itself in order to keep the chamber closed, only when the minimum quantity of ground product is used and the piston is in the position in which it is inserted farthest into the cylindrical chamber and positioned in proximity to the bottom thereof. For all other doses the rotational point of connection between the connecting rod and the crank must stop before bottom dead centre along the path of the connecting rod head.
With such positioning, the admission of the pressurised water into the chamber causes a torque to be produced on the drive shaft and, to compensate the torque, it is necessary to adopt particular technical solutions.
One of these solutions, for example, provides that the motor must be excited in such a way as to apply a greater counter torque in order to ensure the closure of the infusion chamber by the piston.
This solution, however, would require the use of an expensive high torque reduction motor capable of resisting with the motor locked for the entire time of preparation of the beverage without exceeding the admissible heating up limit.
Moreover, with the aforesaid constructional solution, the layer of compressed coffee powder would be prevented from swelling up and therefore from correctly absorbing the water, since the motor would have to exert a torque such as to ensure that the connecting rod reacted with a force greater than that generated by the pressure of the water continuing to compress the layer of coffee powder.
Finally, the fact of using a volume of coffee powder such as to prevent the connecting rod from reaching bottom dead centre raises the problem of how to be able to act on the device for expelling the quantity of exhausted powder from the infusion chamber once the preparation of the beverage is completed.
For example, under these conditions the technical solution illustrated in EP-A-1 306 041 would not be longer practicable.